Quite new to Scribus, however I think it would be nice that in my master page I could specify that left master page X is mirrored in right master page Y. The idea is that inserting a page is currently a very disruptive process.

I'd still have to move the content of the pages since I can't specify that the content is relative to the margins (see how subtle I am about suggesting another way of managing absolute positionning ) But at least, I won't have to go through 50 pages to change the master page each of them is associated with. From my point of view, doing Ctrl-A to select the whole content and moving it by 0.5 cm is relatively easy and involves less clicks then resetting all the master templates.

Both relative and percentage-based positioning/sizing - possibly along with the ability to lock a co-ordinate or dimension to a margin or guide - would be a great addition to Scribus.

Relative and percentage-based positioning/sizing would allow the page size/orientation to be changed without needing much extra work from the user. Think responsive web pages and you've pretty much got it. For example, you could have a yearbook grid laid out in portrait then switch to landscape and the frames would re-position and resize automatically. That's just a basic example.

Locking things to margins/guides would allow the user to change layouts very quickly, automatically moving or resizing frames as the guides move. A great time-saver when experimenting with a new layout.

While both of these things would be great for normal page layout they would also be a gateway for Scribus into the realms of ePublishing.

I can't see anything like this coming soon - if ever - but I definitely think that it should be treated as a possibility for the mid-term future of Scribus.